The invention relates to improvements made to chip cards comprising a card made from a plastics material with a relative elastic bending capacity and encasing a microelectronic data processing chip, a thin microelectronic wafer of semiconductor material additionally being joined to the card of plastics material.
Such a card, made from a plastics material, can withstand a relative degree of bending without breaking. As a result of the reduced dimensions of the chip relative to the dimensions of the card, a certain degree of bending in the card will not, as a rule, cause the chip to rupture and/or become unstuck.
The same does not apply to a very thin microelectronic silicon wafer, which is highly susceptible to breaking and of relatively large dimensions: accordingly, bending of the plastics card will run the risk of this wafer breaking and/or becoming unstuck.
In practice, this problem may arise in particular with finger print sensors: a known chip card is provided with a finger print sensor, which can be connected so as to co-operate with the chip (which generally incorporates a microprocessor), so that the user of the card can be identified by running a comparison of the finger prints detected by the finger print sensor with finger prints previously stored in a memory, in particular in a memory of the microelectronic chip. The silicon wafer constituting a finger print sensor must be large enough for a print of the greater part of the bottom surface of the first joint of a finger (often the index finger) to be applied to it: to be more specific, the dimensions of such a sensor may be approximately 10 to 20 mm by approximately 10 to 20 mm.
A great deal of development work is being done on this type of chip card provided with a finger print sensor, in particular with a view to replacing chip cards operated using a confidential alphanumeric code, for example as access cards or banking cards.
An object of the invention is substantially to propose an improved design of chip card fitted with a thin microelectronic wafer which better meets practical requirements, in particular as regards its strength over time, even if the plastics card is subjected to a relative degree of bending, without significantly increasing the complexity of the manufacturing process or significantly increasing manufacturing costs.
To this end, the invention proposes a chip card of the type described above, wherein:
said wafer is sub-divided into a small number of juxtaposed chips, disposed in a given pattern,
said chips are separated from one another by a predetermined distance which is large enough for each separating strip of plastics material to be subjected to a slight degree of bending without causing adjacent chips to become unstuck and/or break, and
electrical links are established between the chips to provide electrical continuity.
As a result of this arrangement, the joining surface of the thin silicon wafer, which is fragile, is sub-divided and the adjacent chips are separated from one another by separating strips capable of absorbing any bending to which the card is subjected. In order to meet this requirement without imposing any risk, it is desirable for the distance between the facing edges of two chips to be substantially equal to at least 1 mm.
Generally speaking, each chip may be of an approximate rectangular shape with four corners and the chips are disposed with their respective edges substantially parallel. In practice, the chips are aligned in substantially perpendicular rows and columns or alternatively arranged on diagonals.
There is no need for the wafer to be highly sub-divided in order to achieve the desired result: it is sufficient for the chips to be disposed in two to four rows and/or in two to four columns, depending on the size of the chips.
In one example of a preferred application, the thin wafer is a finger print sensor which is electrically connected to the microelectronic chip. This being the case, it is of advantage to provide interpolation meansxe2x80x94for example co-operating with or incorporated in the microelectronic chip and electrically connected to the finger print sensorxe2x80x94capable of reconstructing a full image of finger prints by an interpolation process, including in the zones corresponding to the strips separating the chips; the interpolation algorithm is adapted to the size of the area separating the chips and this dimension must therefore be as small as possible to render detection as faithful as possible. In this context, the space between chips may be between 1 and 1.5 mm, typically about 1 mm, which strikes what appears to be an acceptable compromise between a relative deformability of the card in the spaces between chips and the reliability of the process of detecting and reconstituting finger prints by interpolation.
In a preferred practical embodiment, the finger print sensor is of the capacitive type, incorporating a high plurality of elementary micro-capacitors; each chip incorporates a plurality of such micro-capacitors arranged in a network; and the electrical links between chips provide the electrical continuity between the networks of elementary micro-capacitors belonging to two adjacent chips.